Known is an erroneous refueling prevention device which is provided to a refueling passage in a vehicle such as an automobile, and which rejects the insertion of a refueling gun with a small diameter into the refueling passage, and allows the insertion of a refueling gun with a large diameter into the refueling passage. For example, Patent Literature 1 discloses an erroneous refueling prevention device which, when an insertion pressure of a refueling gun slides sliders to the left and right, unlocks a flap valve from the corresponding slider, and allows the opening operation of the flap valve and the insertion of the refueling gun. The sliders include taper surfaces on their parts with which an end portion of the refueling gun comes into contact, and the taper surfaces serve as a detector for detecting the outer diameter of the refueling gun.